1. Field of the Invention
This invention relates to a method for developing electrostatic latent images and an apparatus for carrying out the method. More particularly, this invention relates to a method and an apparatus for developing an electrostatic latent image with a magnetic brush formed by a developer comprised of a high electrical resistance magnetic toner and a magnetic carrier to obtain a sharp copied image free from fogging.
2. Description
As the magnetic brush developing method, there are known and used two methods, that is, one being a method of development using monocomponent magnetic developer, and the other being a method of development using a two-component magnetic developer. The method of development using one-component developer is advantageous in that the construction of the developing apparatus is simple and its maintenance and repairs can be made readily. In particular, the methods of development using a magnetic toner, so-called, jumping developing method and selective developing method are advantageous in that very sharp copied images can always be obtained at low costs. On the other hand, however, in carrying such methods into effect, it is essential to provide a very close mechanical accuracy to component parts of the developing apparatus, and stating more specifically, it is required, for example, to maintain the clearance between an image carrier and a development sleeve (and the surface of a developer layer formed on the sleeve) over the whole length of the sleeve accurately at a predetermined value. Therefore, it is considerably difficult to meet such requirements in electrophotographic copying systems for forming large-sized copied images such as those of AO size which require the provision of a considerably long sleeve. For this reason, such a system has not yet been developed up to the present time.
Whilst, the method of development using a two-component developer can eliminate the above-mentioned disadvantage inherent to the developing method using monocomponent developer, however, it is disadvantageous in that it is necessary to control always the ratio of mixing toner and carrier such that it is kept in a predetermined range and periodically replace all the developer due to fatigue of the carrier or the like.
Still further, there is known a method of development using a developer comprised of a mixture of a magnetic carrier having a fine particle size and a non-magnetic toner. (see, for example, Japanese Laid-Open Patent Application No. SHO. 59-24416). The method of development disclosed in this publication of Japanese Patent Application comprises the steps of forming a developer brush with a magnetic developer which is comprised of a mixture of an electrically insulating magnetic carrier having a particle size of substantially 5 to 30 .mu.m and an electrically insulating non-magnetic allowing the developer brush to rub the surface of an electrostatic latent image carrier under the influence of an alternating magnetic field to thereby develop the latent image.
According to the above-mentioned method of developing electrostatic latent images using the magnetic carrier of a fine particle size and the magnetic toner, developed images of a very high quality which are free from fogging can be obtained, and also some of the above-mentioned problems can be eliminated, however, there still remains a problem on control of the toner density wherein the ratio of mixing of toner with carrier must be maintained in a range of 6 to 35% by weight.
The present invention has been made in view of the above-mentioned circumstances in the prior art, and has for its object to provide a method for developing electrostatic latent images which eliminates substantially the need for controlling the density of toner and which enables an image having excellent sharpness and enhanced graduation and free from fog to be obtained, and also an apparatus for carrying out the method.